U.S. Pat. No. 6,005,879, issued to Sandstrom et al. on Dec. 21, 1999, entitled PULSE ENERGY CONTROL FOR EXCIMER LASER, the disclosure of which is hereby incorporated by reference, relates to:                A process for controlling pulse energy and integrated energy dose in bursts of pulses produced by an excimer laser. The energy of each pulse in each burst is measured. The rate of change of pulse energy with charging voltage is determined. A pulse energy error is determined for a previous pulse of the present burst. An integrated dose error is also determined for all previous pulses in the current burst. A charging voltage for the next pulse is determined using the pulse energy error, the integrated dose error, the rate of change of energy with charging voltage and a reference voltage. In a preferred embodiment, the rate of change of energy with voltage is determined by dithering the voltage during two pulses of each burst, once lower and once higher. The reference voltage is a voltage calculated using prior energy and voltage data. In this embodiment, the method of determining the reference voltage during a first portion of the pulse is different from the method used during a latter portion of the burst. During the first set of pulses (40 in this embodiment), for each pulse, a specified voltage calculated using voltage and energy data from a corresponding pulse in a previous burst is utilized as a prediction of the voltage needed to produce a pulse energy converging on a target pulse energy. For pulses 41 and thereafter the reference voltage for each pulse is the specified voltage for the previous pulse.        
U.S. Pat. No. 6,034,978, issued to Ujazdowski et al. in Mar. 7, 2000, entitled GAS DISCHARGE LASER WITH GAS TEMPERATURE CONTROL, the disclosure of which is hereby incorporated by reference, relates to:                A gas discharge laser with fast response gas temperature control to maintain laser gas temperature within desired limits during burst mode operation. Preferred embodiments include a passive temperature stabilizer . . . . Preferred embodiments utilize heating elements and coolant flow control to regulate laser gas temperatures using processors programmed to anticipate idle periods.        
U.S. Pat. No. 6,317,447, issued to Partlo et al. on Nov. 13, 2001, entitled ELECTRIC DISCHARGE LASER WITH ACOUSTIC CHIRP CORRECTION, the disclosure of which is hereby incorporated by reference, relates to:                Methods and structural changes in gas discharge lasers for minimizing wavelength chirp at high pulse rates. Applicants have identified the major cause of wavelength chirp in high pulse rate gas discharge lithography lasers as pressure waves from a discharge reflecting back to the discharge region coincident with a subsequent discharge . . . . During burst mode operation, the laser gas temperature . . . changes . . . over periods of a few milliseconds . . . changing . . . the location of the coincident pressure waves from pulse to pulse within the discharge region causing a variation in the pressure of the laser gas which in turn affects the index of refraction of the discharge region causing the laser beam exiting the rear of the laser to slightly change direction. This change in beam direction causes the grating in the LNP to reflect back . . . a slightly different wavelength causing the wavelength chirp. Solution to the problem is to include in the laser chamber structural elements to moderate or disperse the pressure waves and to maintain the laser gas temperature as close as feasible to constant values.        
U.S. Pat. No. 6,529,531, issued to Everage et al. on Mar. 4, 2003, entitled, FAST WAVELENGTH CORRECTION TECHNIQUE FOR A LASER, the disclosure of which is hereby incorporated by reference, relates to:                [An e]lectric discharge laser with fast chirp correction . . . includ[ing] at least one piezoelectric drive and a fast wavelength detection means and . . . a feedback response time of less than 1.0 millisecond. In a preferred embodiment a simple learning algorithm . . . allows advance tuning mirror adjustment in anticipation of the learned chirp pattern. Techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver. In another preferred embodiment chirp correction is made on a pulse-to-pulse basis where the wavelength of one pulse is measured and the wavelength of the next pulse is corrected based on the measurement.        
U.S. Pat. No. 6,532,247, issued to Spangler et al. Mar. 11, 2003 LASER WAVELENGTH CONTROL UNIT WITH PIEZOELECTRIC DRIVER, the disclosure of which is hereby incorporated by reference, relates to:                An electric discharge laser with fast wavelength correction. . . . Techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver for tuning the laser wavelength using a tuning mirror. A preferred control technique is described (utilizing a very fast wavelength monitor) to provide the slow and intermediate wavelength control and a piezoelectric load cell in combination with the piezoelectric driver to provide the very fast (few microseconds) wavelength control.        
U.S. Pat. No. 6,567,450, issued to Myers et al. on May 20, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, relates to                An injection seeded modular gas discharge laser system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 ml or greater. Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. The chambers can be controlled separately permitting separate optimization of wavelength parameters in the master oscillator and optimization of pulse energy parameters in the amplifying chamber. A preferred embodiment in an ArF excimer laser system configured as a MOPA and specifically designed for use as a light source for integrated circuit lithography. In the preferred MOPA embodiment, each chamber comprises a single tangential fan providing sufficient gas flow to permit operation at pulse rates of 4000 Hz or greater by clearing debris from the discharge region in less time than the approximately 0.25 milliseconds between pulses. The master oscillator is equipped with a line narrowing package having a very fast tuning mirror capable of controlling centerline wavelength on a pulse-to-pulse basis at repetition rates of 4000 Hz or greater to a precision of less than 0.2 pm.        
U.S. Pat. No. 6,690,704, issued to Fallon et al. on Feb. 10, 2004, entitled CONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGE LASER, the disclosure of which is hereby incorporated by reference, relates to:                [A] control system for a modular high repetition rate two discharge chamber ultraviolet gas discharge laser . . . with a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. Feedback timing control techniques are provided for controlling the relative timing of the discharges in the two chambers with an accuracy in the range of about 2 to 5 billionths of a second even in burst mode operation.        
U.S. Pat. No. 6,625,191, issued to Knowles et al. on Sep. 23, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, the disclosure of which is hereby incorporated by reference, relates to:                An injection seeded modular gas discharge laser system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 mJ or greater. Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. The chambers can be controlled separately permitting separate optimization of wavelength parameters in the master oscillator and optimization of pulse energy parameters in the amplifying chamber.        
U.S. Pat. No. 6,650,666, issued to Spangler et al. on Nov. 18, 2003, entitled LASER WAVELENGTH CONTROL UNIT WITH PIEZOELECTRIC DRIVER, the disclosure of which is hereby incoroprated by reference, relates to:                An electric discharge laser with fast wavelength correction. Fast wavelength correction equipment includes at least one piezoelectric drive and a fast wavelength measurement system and fast feedback response times. . . . Preferred embodiments provide (1) fast feedback control based on wavelength measurements, (2) fast vibration control, (3) active damping using the load cell and an active damping module, (4) transient inversion using feed forward algorithms based on historical burst data. A preferred embodiment adapts the feed forward algorithms to current conditions. Another preferred embodiment measures tuning mirror position to permit wavelength pretuning and active wavelength tuning.        
U.S. Pat. No. 6,192,064, issued to Algots et al. on Feb. 20, 2001, entitled NARROW BAND LASER WITH FINE WAVELENGTH CONTROL, the disclosure of which is hereby incorporated by reference, relates to:                A smart laser having automatic computer control of pulse energy, wavelength and bandwidth using feedback signals from a wavemeter. Pulse energy is controlled by controlling discharge voltage. Wavelength is controlled by very fine and rapid positioning of an RMAX mirror in a line narrowing module. Bandwidth is controller by adjusting the curvature of a grating in the line narrowing module. Preferred embodiments include automatic feedback control of horizontal and vertical beam profile by automatic adjustment of a prism plate on which beam expander prisms are located and automatic adjustment of the RMAX tilt. Other preferred embodiments include automatic adjustment of the horizontal position of the laser chamber within the resonance cavity. In preferred embodiments, feedback signals from a wavelength monitor are used to position the RMAX mirror. In other preferred embodiments a separate laser beam reflected off the RMAX mirror on to a photodiode array is used to position the mirror.        
U.S. Pat. No. 6,621,846, issued to Sandstrom et al. on Sep. 16, 2003, entitled ELECTRIC DISCHARGE LASER WITH ACTIVE WAVELENGTH CHIRP CORRECTION, the disclosure of which is hereby incorporated by reference, relates to:                [An e]lectric discharge laser with active chirp correction. This application discloses techniques for moderating and dispensing . . . pressure waves. In some lasers small predictable patterns remain which can be substantially corrected with active wavelength control using relatively slow wavelength control instruments of the prior art. In a preferred embodiment a simple learning algorithm is described to allow advance tuning mirror adjustment in anticipation of the learned chirp pattern. Embodiments include stepper motors having very fine adjustments so that size of tuning steps are substantially reduced for more precise tuning. However, complete elimination of wavelength chirp is normally not feasible with structural changes in the laser chamber and advance tuning; therefore, Applicants have developed equipment and techniques for very fast active chirp correction . . . includ[ing] a combination of a relatively slow stepper motor and a very fast piezoelectric driver. In another preferred embodiment chirp correction is made on a pulse-to-pulse basis where the wavelength of one pulse is measured and the wavelength of the next pulse is corrected based on the measurement.        
U.S. Pat. No. 6,721,340, issued to Fomenkov et al. on Apr. 13, 2004, entitled BANDWIDTH CONTROL TECHNIQUE FOR A LASER, the disclosure of which is hereby incorporated by reference, relates to:                A technique for bandwidth control of an electric discharge laser. Line narrowing equipment is provided having at least one piezoelectric drive and a fast bandwidth detection means and a bandwidth control having a time response of less than about 1.0 millisecond. In a preferred embodiment wavelength tuning mirror is dithered at dither rates of more than 500 dithers per second within a very narrow range of pivot angles to cause a dither in nominal wavelength values to produce a desired effective bandwidth of series of laser pulses.        
U.S. Pat. No. 6,078,599, issued to Everage et al. on Jun. 20, 2000, entitled WAVELENGTH SHIFT CORRECTION TECHNIQUE FOR A LASER, the disclosure of which is hereby incorporated by reference, relates to:                A wavelength shift correction system for a laser system is provided for correcting wavelength chirps . . . includ[ing] a learning algorithm that learns characteristics of a wavelength chirp from a laser and a computer system that executes the learning algorithm and provides wavelength correction control signals based on the learned characteristics to reduce the magnitude of the wavelength shift of the present wavelength chirp and subsequent wavelength chirps.        
U.S. Pat. No. 6,735,225, issued to Albrecht et al. on May 11, 2004, entitled CHIRP COMPENSATION METHOD AND APPARATUS, the disclosure of which is hereby incorporated by reference, relates to:                A wavelength chirp compensation method for an excimer or molecular fluorine laser system operating in burst mode, includes pre-programming into a computer of the laser system resonator tuning optic adjustments for making the adjustments during pauses between bursts to compensate wavelength chirp at beginnings of succeeding bursts.        
Applicants have observed in the operation of certain gas discharge lasers, e.g., ArF, KrF and molecular fluorine gas discharge lasers, including a lasing medium-creating gas comprising, e.g., fluorine and another gas, e.g., krypton or argon, and a buffer gas or gases, e.g., neon and helium and also in configurations including two chambers, e.g., master oscillator-power amplifier (“MOPA”), master oscillator-power oscillator (“MOPO”), power oscillator-power amplifier (“POPA”) and power oscillator-power oscillator) and including even other forms of multi-chambered lasers systems where the chambers may be mounted on a single frame and/or optically interconnected by optics that are physically attached in some way to each chamber, that slow wavelength transients can occur under a variety of circumstances. Applicants have found that such transients are generally burst correlated in laser systems that provide laser output light pulse beams in bursts of pulses per laser output light beam, e.g., one hundred of so pulses at pulse repetition rates varying from, e.g., 2 kHz and above up to, e.g., about 6 kHz or even higher (i.e., with a 500 μs to 167 μs time period or less between pulses) with some down time, e.g., on the order of milliseconds or more between bursts. In addition, other optical elements, e.g., with a line narrowing module (“LNM”) which may contain moving parts, e.g., a fast tuning optical element, e.g., a fast tuning mirror, may add vibrational disturbances to the system and may include, e.g., certain resonant frequencies, which, with less than perfect vibrational isolation between laser system modules can impact the occurrence of such transients.
Such transients can be significant, e.g., as much as 0.2 pm in amplitude, which can, under the strict requirements for wavelength pulse to pulse stability and wavelength sigma during a burst, result in the output laser light pulse beam produced by the laser system being out of specification, when operating, e.g., at certain pulse repetition rates where the transients are most significant.
Applicants have found that such transients can take about 100 shots within a single burst, usually starting at pulse one or close to pulse one to build to full amplitude. Typically, also, applicants have found, such transients occur within relatively narrow repetition rate bands.
It is applicants' belief that such transients occur for a variety of reasons, including one which applicants have observed at or around 1700 Hz which most likely is due to mechanical vibrations in the system, e.g., in the LNM and at or around 1850 Hz, which most likely is due to acoustic resonances in a chamber, e.g., in one or both of the chambers in a two chambered system, or only in one, e.g., the MO chamber in, e.g., a MOPA system configuration.
Applicants have further examined the root causes of the inability to deal properly with these transients, and propose in the present application aspects of an embodiment of the present invention relating to such solution.